1. Field of the Invention
The present invention concerns a fuel cell system.
2. Detailed Description of the Prior Art
Of latest years, a fuel cell system, comprising a reformer for reforming a hydrocarbon base fuel including natural gas, city gas, methanol, LPG, butane and so on as raw fuel into hydrogen, a CO transformer for transforming carbon monoxide, a CO eliminator for removing carbon monoxide, a fuel cell for generating electricity from hydrogen, and a burner for reformer for burning hydrogen gas discharged from the fuel cell and supply the reformer with heat necessary for reforming reaction, or a fuel cell system as small power supply comprising further a process gas burner burning hydrogen until each reactor stabilizes during the start or stop, have been proposed.
FIG. 5 is a system diagram showing a fuel cell system of the prior art.
A fuel cell system GS using a fuel cell 6 comprises a heat recovery unit RD in addition to the fuel cell 6.
The heat recovery unit RD is linked by a hot water circuit or the like comprising a hot water storage tank 50, heat exchangers 32, 46, 71 and pumps 33, 47, 72.
The fuel cell 6 is provided with a fuel gas supply unit comprising a desulfurizer 2, a reformer 3, a CO transformer 4, a CO eliminator 5 and so on, a reaction air supply unit comprising an air pump 11, a water pump 21 and so on, electrodes such as fuel electrode 6a, air electrode 6k and so on, and a refrigeration unit of the fuel cell 6 comprising a water tank 21, a pump 48, a refrigeration section 6c and so on.
Electricity generated by the fuel cell 6 is boosted by a DC/DC converter and connected to the main power through a distribution system link inverter (not shown). On the other hand, the power is supplied from here as power of other electric apparatus such as lighting, air conditioner, in a house, office, and so on.
The fuel cell system GS using such a fuel cell 6 intends to use efficiently the energy held by the fuel used for the fuel cell 6, for example, by producing hot water from the city water using heat generated during the power generation by the fuel cell 6 at the same time as the power generation, storing the hot water in a hot water storage tank 50 and supplying to the bath, kitchen and so on.
In the fuel gas supply unit of the aforementioned fuel cell system GS, natural gas, city gas, methanol, LPG, butane or other raw fuel 1 supplied to a desulfurizer 2, and here, sulfur component is removed form the raw fuel.
When the raw fuel having passed through the desulfurizer 2 is boosted up by a booster pump 10 and delivered to the reformer 3, hot water is sent from a water tank 21 through a water pump 22 and heated by the heat exchanger 17 to generate water vapor, and they meet before being supplied. A reformed gas containing hydrogen, carbon dioxide, and carbon monoxide is produced in the reformer 3. Gas having passed through the reformer 3 is supplied to the CO transformer 4, and here, carbon monoxide contained in the reformed gas is transformed into carbon dioxide. Gas having passed through the CO transformer 4 is supplied to the CO eliminator 5 and here, untransformed carbon monoxide in the gas having passed through the CO transformer 4 is reduced, for instance, equal or inferior to 10 ppm (volume) and aqueous gas (reformed gas) of high hydrogen concentration is supplied to a fuel electrode 6a of the fuel cell 6 through a pipe 64.
At this time, the moisture addition amount to the reformed gas is adjusted by regulating the amount of hot water supplied from the water tank 21 to the reformer 3.
The reaction air supply unit humidifies by supplying air from the air pump 11 to the water tank 21, and delivering to a gas phase 53 all the way frothing up reaction air in the hot water in the water tank 21.
Thus, the reaction air after humidification so as to maintain the reaction properly in the fuel cell 6 is supplied to the air electrode 6k of the fuel cell 6 from the water tank 21 through the pipe 25.
In the fuel cell 6, power is generated by electrochemical reaction of hydrogen in the reformed gas supplied to the fuel electrode 6a and oxygen in the air supplied to the air electrode 6k though the gas phase 53 of the air pump 11 and the water tank 21.
The refrigeration unit of the fuel cell 6 is a refrigeration unit placed side by side with the electrodes 6a, 6k of the fuel cell 6, in order to prevent the fuel cell 6 from overheating due to reaction heat or the like of the electrochemical reaction, and designed to circulate hot water of the water tank 21 in a refrigeration section 6c as cooling water by a pump 48, and controls to maintain the temperature in the fuel cell 6 to a temperature appropriate for power generation (for instance, to the order of 70 to 80° C.) by the cooling water.
The chemical reaction in the reformer 3 being endoergic, it has a burner for reformer 12 to sustain the chemical reaction by heating, which is supplied with raw fuel through the pipe 13, supplied with unreacted hydrogen passing through the fuel electrode 6a, through the pipe 15, and supplied with air through a fan 14. When the present fuel cell system GS starts, raw fuel is supplied to the burner for reformer 12 through the pipe 13 and burned, and when the temperature of the fuel cell 6 stabilizes, after the starting, the supply of raw fuel from the pipe 13 is reduced, and unreacted hydrogen (off-gas) discharged from the fuel electrode 6a is supplied through the pipe 15 to sustain the combustion.
On the other hand, the chemical reaction in the CO transformer 4 and the CO eliminator 5 is exothermic. The refrigeration control is performed, during the operation, so that the temperature does not exceed the reaction temperature by the heat of the exothermic reaction. Thus, these predominated chemical reaction and power generation are sustained in reformer 3, the CO transformer 4, the CO eliminator 5 and the fuel cell 6.
Heat exchangers 18, 19 are connected respectively between the aforementioned reformer 3 and the CO transformer 4, and, the CO transformer 4 and the CO eliminator 5.
And, how water of the water tank 21 circulates in respective heat exchangers 18, 19 through the pumps 23, 24, and these hot waters cool respectively the gas having passed through the reformer 3 and the CO transformer 4. Though not illustrated, it is also possible to connect a heat exchanger between the CO eliminator 5 and the fuel cell 6 for cooling gas having passed through the CO eliminator 5.
A heat exchanger 17 is connect to an exhaust system 31 of the aforementioned reformer 3 and, when hot water from the water tank 21 is supplied through the pump 22, it is vaporized by the heat exchanger 17, and the water vapor is mixed with the raw fuel and supplied to the reformer 3.
The present fuel cell system GS is provided with a process gas burner (PG burner) 34.
When the fuel cell system GS starts, the composition of the reformed gas having passed through the reformer 3, CO transformer 4 and CO eliminator 5 has not attained a stable specified value appropriate for operating the fuel cell 6, and it is impossible to supply the fuel cell 6 with the gas, until it stabilizes. There, until the stabilization of respective reactor, the gas whose gas composition has not attained the specific value is introduced in the PG burner 34 and burned.
37 is a fan for delivering combustion air to the PG burner 34.
Then, when respective reactor stabilizes and the CO concentration in the gas has attained the specific value (for instance, equal or inferior to 10 ppm (volume)), it is introduced in the fuel cell 6 for power generation. Unreacted gas that could not be used for power generation in the fuel cell 6 is introduced initially in the PG burner 34 and burned and, when the temperature of the fuel cell 6 stabilizes, off-gas from the fuel cell 6 is introduced in the burner 12 of the reformer 3 through the pipe 15 and burned.
In other words, after the start of the fuel cell system GS, an on-off valve 91 is closed until the temperature stabilization of respective reactor, and the reformed gas is supplied to the PG burner 34 through a pipe line 35 and an on-off valve 36.
In case of temperature stabilization of respective reactor, now the on-off valve 91 is opened while the on-off valve 92 is closed, until the temperature of the fuel cell 6 stabilizes in a temperature range near the operation temperature (for instance 70 to 80° C.), and the reformed gas is supplied to the PG burner 34 through a pipe line 38 and an on-off valve 39, and burned there.
In case where the temperature of the fuel cell 6 stabilizes at the operation temperature, and power is generated continuously, the on-off valves 91, 92 are opened while the on-off valves 36, 39 are closed, and unreacted gas (off-gas) having passed through the fuel cell 6 is supplied to the burner for reformer 12 through a pipe line 15.
The hot water storage tank 50 is supplied with city water though a water pipe 61. The city water supplied to the hot water storage tank 50 is heated by exhaust heat generated from the fuel cell system GS, and the hot water risen in temperature is supplied outside through a hot water supply pipe 62.
For instance, another heat exchanger 32 is further connected to the exhaust system 31, in addition to the heat exchanger 17, and water of the hot water storage tank 50 circulates in the heat exchanger 32, through a pump 33, for exhaust heat recovery.
Also, a heat exchanger 46 is connected to an exhaust system 45 of the PG burner 34 and, water of the hot water storage tank 50 circulates in the heat exchanger 46, through a pump 47, for heat recovery.
Water returning through the heat exchangers 18, 19 by the pumps 23, 24, 48 and cooling water circulating in the refrigeration section 6c of the fuel cell 6 flow in the water tank 21 through a water pipe 73 and, on the other hand, a water supply unit 68 for supplying the water tank 21 with water is connected.
The water supply unit 68 is comprised of an electric motor operated valve 56, a supply tank 67, a pump 74 and so on. The supply tank 67 is a tank for storing temporarily through a pipe 70 water generated from a city water supply unit 69 and the fuel cell 6 for permitting to supply the water tank 21 with water.
Water generated from the fuel cell 6 includes, for instance, drain water obtained by conducting gas discharged from the air electrode 6k of the fuel cell 6 into the heat exchanger 71 and cooling the inside of the heat exchanger 71 with water circulating between the hot water storage tank 50 by the pump 72 or water contained in the gas discharged from the fuel electrode 6a. 
The city water supply unit 69 is connected to a water source 78 through a water pipe 52 having an electric motor operated valve 76 and when a water level gauge 79 detects the lowering of water level due to the reduction of water quantity in the supply tank 67, a liquid level control unit 77 opens the electric motor operated valve 76, and replenishes the supply tank 67 with water through the water pipe 52 and a water treatment unit (ion exchange resin) 51 using the water pressure of the water source 78, in order to retain a water quantity that would not impede the supply of the water tank 21 with water.
The water tank 21 has a liquid level control unit LC for keeping the level of water so as to form an air portion (gas phase) 53 all the time in the upper part in the tank and a temperature control unit TC for keeping the water temperature in the water tank 21 within the set range.
The liquid level control unit LC comprises a water level gauge 54 and a control unit of an electric motor operated valve 56 for monitoring the water quantity in the water tank 21 all the time, stores water in the tank so that reaction air is humidified moderately when it passes in the water tank 21 and supplied to the fuel cell 6, and controls the water quantity so as to form the gas phase 53 in the upper part, operates the pump 74 in case of lowering of the water level, introduces treated water from the supply tank 67 through a pipe 84 by adjusting the opening of the electric motor operated valve 56, for keeping the water level in the water tank 21 within the set range.
55 is a wave-eliminating board for preventing the level detection by the water level gauge 54 from becoming unstable due to frothing and so on.
The temperature control unit TC is an unit for keeping the water temperature in a temperature range of, for instance, 60 to 80° C. (set temperature) so as to realize a moderate humidification in the water tank 21, when the air electrode 6k of the fuel cell 6 is supplied with reaction air. 63 is a perforated board for bubbling.